Phlegethos is a fiery wasteland filled with active volcanoes, hills of ash, gouting fires, streams of magma, pits of smoking excrement, and burning sand beneath a screaming rain of magical fire flakes. Lady Fierna and Archduke Belial are the lords of this layer. Few can exist here for long without suitable protection from the extreme heat. The fortress capital city of Abriymoch is built on the lip of a volcano and composed of hardened magma in the caldera of a volcano and is somewhat more bearable than the rest of Phlegethos. It has little tolerance for outsiders, but important allies of Fierna, the Lord of the Fourth, have their places in its society. The pit fiend Gazra, commander of the hamatulas, dwells here in a palace of crystal. Fierna and Belial reside in a palace of pure obsidian high up on one side of the city.
The ground of Phlegethos is always uncomfortably hot. This, as well as patrols of barbed devils, keep most intruders constantly on the move. Tremors are common, and it is not unusual for the ground to violently erupt or fissures to open without notice. The sky of Phlegethos is a dark, starless void, but constantly leaping flames provide weird illumination to the harsh landscape.
Rivers of liquid fire feed and emerge from at least two massive lakes where the flames burn brightest. This fiery “water” is inhabited by numerous wandering salamanders that were brought to the layer in days long past, to be bound in servitude to the master of the plane. Given that salamanders are not prone to taking orders, this didn’t work out, and most were slain by Belial’s servants and vassals. Those few who escaped remain bound to Phlegethos by Belial’s magic, and continue to survive by avoiding large groups of baatezu and snatching lone individuals who venture too close to one of the fiery rivers.
The Pit of Flame is a boiling, fiery sea of refuse empowered by the primal energy of Baator. It is a place of both promotion and punishment for baatezu across the nine circles. Thousands of osyluths watch it to ensure that it is not abused.
Most of Phlegethos is inhabited by barbed devils, though other baatezu live there as well.
In the fiery plains beyond Abriymoch, the Sumerian goddess Inanna lives in a realm called the Jealous Heart.